Strain gages must generally be protected against the penetration of moisture, to avoid undesirable influences on the measurement signal being caused by humidity or other environmental dampness. Conventional strain gages, per se, are ordinarily not sufficiently protected against moisture. To achieve the necessary protection, the strain gages are first mounted on a measuring transducer or on some other component of which the strain is to be measured, and are then either coated with a moisture-protective layer or are completely encapsulated by a bellows arrangement, for example.
German Patent Laying-Open Document 2,728,916 discloses a typical conventional strain gage of the above described general type. The strain gage is mounted on a measuring transducer and is then coated with an electrically insulating coating layer, for example, a resin layer. The insulating layer is applied in such a manner that it extends beyond the edges of the strain gage device and thus also coats a portion of the transducer member around the edges of the strain gage. A metal layer is then applied onto the insulating layer and similarly overlaps and covers a portion of the transducer all around the edges of the strain gage. The electrical contacts of the strain gage extend under this encapsulating cover through the interior of the transducer and pass to the outside in a sealed manner.
Thus, according to German Patent Laying-Open Document 2,728,916, the moisture-tight hermetic encapsulation is not interrupted or broken at any location. However, this manner of encapsulating or coating the strain gage can only be used for strain gages that have already been mounted on a transducer or other component of which the strain is to be measured. Moreover, the conventional manner of encapsulating or covering the strain gage necessitates additional work steps to be carried out after applying the strain gage onto a component, which slows down final assembly and mounting, and which can lead to poor encapsulation results if the final covering step is carried out under adverse conditions in a field installation.
European Patent Laying-Open Document 0,107,966 and corresponding U.S. Pat. No. 4,557,150 (Utsunomiya) disclose a strain gage having a foil- or film-type covering including an insulating layer and an aluminum layer. Similarly as described above, the disclosed covering extends beyond the strain gage and thus covers a portion of the transducer all around the edges of the strain gage. According to one embodiment of U.S. Pat. No. 4,557,150, in order to achieve a hermetic encapsulation and thereby prevent the penetration of moisture along the perimeter edges of the foil- or film-type covering, an aluminum layer is deposited along the perimeter edges of the covering. Thus, the disclosed manner of covering or encapsulating a strain gage can only be used for strain gages that have already been applied onto a transducer member, such as a load cell.
European Patent Laying-Open Document 0,460,249 discloses a strain gage having a metallic frame surrounding the resistor grid together with its electrical contacts. The space within the frame is filled with an insulating material and is covered by a metal foil that is attached to the frame. This known method of encapsulation is relatively complicated and thus cost intensive due to the number of complicated operations required to form the encapsulation.
U.S. Pat. No. 3,274,528 (Bermann) discloses a strain gage in which the measuring resistor grid is completely embedded in a so-called adhesive matrix comprising a polymerizable base material. A continuous process is used to manufacture a strip of a plurality of the strain gages interconnected to one another in a row along the strip. The individual strain gages are separated from one another before they are applied onto a component. Once a strain gage has been applied onto the component, no further covering or encapsulating is carried out. However, only a limited protection against the penetration of moisture is achieved, because the encapsulation is provided solely by embedding the resistor grid within the synthetic matrix. A complete satisfactory protection against moisture penetration is not achieved because a metallic covering layer is not provided over the embedded strain gage. Thus, the penetration of moisture can still influence the measurement results.
U.S. Pat. No. 3,315,200 (Hannay) discloses a strain gage including a resistor grid sandwiched with adhesive between the oxide covered inner surfaces of two metal foils. Each of the metal foils is first prepared by forming a non-conducting film such as an oxide layer on the surface of the metal foil that will face inwardly. Then, an adhesive layer is applied on the oxide films. The resistor grid is formed on a backing film, and then pressed against one of the adhesive layers. Thereafter the backing film is stripped off and the adhesive layer applied to the second metal foil is pressed against the back side of the resistor grid. Thus, the finished strain gage does not include a resistor grid on a backing film, but rather has a naked resistor grid sandwiched between adhesive layers and outer covering metal foils.
Several disadvantages can exist in the prior art strain gage according to U.S. Pat. No. 3,315,200. For example, the resistor grid can be damaged when stripping off the backing film. Furthermore, because the two outer metal foils forming the cover layers are apparently individually precut to size and then pressed together to form the finished sandwich, misalignments of the edges of the several layers can occur. Moreover, this prior art does not address the problem of moisture penetration and is not directly concerned with such a problem. Instead, the reference is directed to a strain gage that is to be attached to a component having a high fluctuating temperature, for which the problems of moisture penetration are generally known to be less important.